The situation has not improved; it is, in fact, notably worse. If the cats are both out, Tori hisses and growls at Lexi, making noises I did not know a cat could make, and rather than leaving well enough alone, Lexi keeps trying to make overtures toward her. Whichever cat I put in the bathroom freaks out -- we tried to leave Lexi in there overnight, and then learned at 3:30am that he knows how to open the door. After enough caterwauling, we herded Tori into the bathroom and had some relative peace. This morning we attempted to reunite them, an effort which went poorly, so now Lexi is back in there (with something heavy blocking the door, which should keep him from being able to push it open). I barely slept and really don't know what to do now.
Everything on the Internet said to keep them separated, switching rooms regularly so they can still smell each other, and then slowly reintroduce them, a process which can take days. But I'm going out of town on Thursday; we don't have days. Any advice is welcome. :\
Update: On advice from my friend A, I let Lexi out while watching both kitties like a hawk, and after some hissing and posturing, everyone settled down and we seem to have relative peace. I'm still wary, but the situation is stable anyway. Let's hope they work this out on their own now, another night like last one is not really on my wishlist.